Despite the worldwide distribution of rickettsial diseases and the highly pathogenic nature of Rickettsiae, there is a substantial gap in our knowledge about their mechanisms of pathogenesis. Although rickettsiae offer a fascinating model of intracellular parasitism, there are significant problems associated with studying an obligate intracellular parasite. The rickettsiae are not amenable to sophisticated studies due to a lack of genetic tools, which has resulted in slow progress in correlating rickettsial genes and gene function. The long-term goal of this proposal is to select and characterize R. prowazekii genes that contribute to virulence and pathogenesis, and to identify one or more gene products as candidate targets for vaccine against typhus group rickettsiae. This goal will be achieved via selection, cloning and the expression of genes encoding R. prowazekii virulence-associated proteins, and their use in immunoprotection against typhus group rickettsiae (R. prowazekii and R. typhi). Experiments will be performed to identify and characterize R. prowazekii genes encoding proteins for specific use in irnrnunoprotection against typhus group rickettsiae (R. typhi and R. prowazekii). Rickettsial homologs of the genes involved in cell division, adhesion, and invasion of host cells will be selected from R. prowazekii genome sequence, and gene expression studies as well as functional analyses will be carried out. In addition, we will generate R. prowazekii and R. typhi mutants lacking functional targeted genes. These mutants will be used for functional analysis and their inclusion in developing attenuated non-virulent strains for a broad-based protective vaccine against pathogenic rickettsiae.